This invention relates to methods for decomposing ozone. In particular it provides means for contacting an ozone-containing gas with a material which eliminates residual ozone, such as that amount present in offgas from contactor-type reactors.
Ozone is used commercially in a variety of oxidation processes, including disinfection of potable water, deodorization of air and waste gases, preservation of perishible goods, chemical synthesis and in the treatment of industrial or municipal wastes. Many of these processes involve contacting an ozone-containing gas with an aqueous reactant. Since ozone is often used in stoichiometric excess or reacts incompletely, residual ozone in a moist offgas is an effluent. Prolonged contact with ozone in a concentration of more than 1-10 ppm is considered unsafe, and venting the residual ozone directly in the off-gas to the atmosphere might prove hazardous. For this reason, a decomposer to kill the remaining ozone is employed in conjunction with most ozone treatment processes.
Ozone is known to decompose thermally in the range from about 80.degree. C to 500.degree. C. It is generally understood that ozone undergoes decomposition to diatomic oxygen (O.sub.2) and nascent oxygen [O] in contact with another body. The nascent oxygen then reacts with another molecule of ozone (O.sub.3) to yield another two molecules of oxygen (O.sub.2).
The decomposition of ozone is catalyzed by many materials. For instance, solid beds of NaOH, Ag.sub.2 O, NiO, MnO.sub.2 and natural or synthetic zeolites are known to accelerate ozone decomposition at various operating temperatures.
The preferred decomposition systems are those which are effective at ambient temperature, thereby avoiding the energy costs of heating the entire offgas stream to kill a minor amount of toxic ozone. While a number of solid catalytic materials are available, their high cost or short life make many catalysts uneconomical. Where the overall process involves ozone contacting a water-containing stream, as in the treatment of aqueous waste-streams, moisture taken up by the effluent gas stream can have a deleterious affect on the decomposition catalyst. This has been a particularly difficult problem where zeolite molecular sieve materials have been used for decomposing ozone. It is generally believed that ozone is adsorbed by the crystalline alkali metal aluminosilicates. According to U.S. Pat. Nos. 3,006,153 and 3,663,418, if the temperature is maintained below -50.degree. C, no substantial amount of decomposition takes place in the solid molecular sieve structure. However, at higher temperatures, oxygen is released in a rapid decomposition. Commercial availability and low cost makes zeolite structures attractive; however, when the stream-contained residual ozone also contains water, the strong affinity of the molecular sieve for its hydration state renders the bed practically non-reactive for ozone decomposition. This is probably due to occupation of adsorption sites by water. In the prior art, it was necessary to interrupt ozone flow through the decomposition reactor for long periods to permit regeneration of the molecular sieve by heating to relatively high temperatures (e.g., 200.degree. C to 300.degree. C). This required multiple beds for continuous service, with a spare reactor being switched into service while the moisture-laden zeolite was being reactivated.